1. Field of the Invention
The present invention relates to an antenna system for wireless communication devices, and more particularly to a simplified, low cost antenna system providing spatial diversity to combat multipath effects in communication systems.
2. Description of Related Art
Local area networks (LAN) are used in the wireless transmission and reception of digitally-formatted data between sites within a building, between buildings, or between outdoor sites, using transceivers operating at frequencies in the range 2.4-2.5 GHz., 5.2-5.8 GHz., and others. Antennas operating over these frequency bands are required for the transceivers in LAN devices. A LAN structure permits many devices, such as computers, to communicate with each other or with other devices such as servers or printers. The individual stations in a LAN may be randomly positioned relative the other stations in the LAN, therefore an omnidirectional antenna is often required for the LAN""s transceivers. One drawback of an omnidirectional antenna is its susceptibility to multipath interference which can reduce signal strength by phase cancellation. This may result in unacceptable error rates for the digital information being transferred over a LAN.
In many wireless systems it is necessary to employ some form of antenna diversity to combat multipath effects in the communication system. The antenna diversity can be accomplished in the form of frequency diversity, time diversity, or spatial diversity. In frequency diversity, the system switches between frequencies to combat multipath interference. In time diversity systems, the signal is transmitted or received at two different times. In spatial diversity systems, two or more antennas are placed at physically different locations to combat multipath interference.
Many prior art systems use a pair of ceramic patch antennas to form a spatially diverse antenna configuration. A ceramic patch antenna typically includes a ceramic substrate, a metalized patch formed on one surface of the substrate, and a ground plane disposed on the opposite surface of the substrate. A feed hole couples the metallized patch to the receiver/transmitter. The use of high dielectric constant materials for the ceramic substrate results in an antenna which is physically small. However, ceramic patch antennas tend to be relatively expensive. Furthermore, connecting the antenna to a low cost circuit board often requires special connectors and cabling, which add cost to the system.
A compact diversity antenna system for use with a communication system such as a LAN (local area network) is described. The antenna system consists of two moderately directional arrays disposed back-to-back, with separate rf feed ports for each array. The construction of the arrays is unique in the use of a common reflector element with two driven elements. Further, the driven elements are compact, and provide electrical performance nearly equal to full-size elements. The antenna volume has been minimized, making the antenna suitable for internal or external mounting on LAN devices. The antennas are formed by conductive traces on a first major surface of a dielectric substrate, such as a printed wiring board. Balun/feed networks are provided on a second, parallel major surface of the substrate. The balun traces are microstrip transmission lines using the wide reflector element trace on the first surface as a ground plane.
The antenna of the present invention provides two rf ports, each connected to a moderately directional antenna. The two patterns of the antennas effectively isolate azimuth sectors of 180 degrees, with maximum isolation to the rear of an array and maximum gain to the front of an array. In this way appropriate circuitry in a LAN device""s transceiver can switch between antenna ports and select the antenna with maximum signal strength. Multipath signals coming from directions other than that of the strongest signal will be attenuated.
Additional objects of the antenna system according to the present invention include the provision of a compact, low cost antenna fabricated on a printed circuit board.
Other aspects and advantages of the invention are disclosed upon review of the figures, the detailed description, and the claims which follow.